The indigo will work on all natural fibers, protien and plant. The nylon won't hold onto the indigo well although it would probably take it to some extent. Although I don't know this for sure I suspect it has to do with the size of the indigo molecules.Have you looked at the info at our Pre-reduced Indigo page?This might give you a little more info. Check at Paula Burch's site also, as she has a very informative page on natural dyes.Hope this helps,

The problem with indigo dye on synthetic fibers as that the synthetic fibers do not have pores into which the indigo dye molecules can become lodged.

The way indigo works is that it is water-soluble when it is in the reduced form, and can seep into the spaces betwen fiber molecules in any natural fiber. Then, when you oxidize it by exposing it to air, the indigo becomes insoluble in water, so any that is inside the fiber strands at the time becomes stuck. (The mechanism is the same for all vat dyes.)

As a general rule, natural fibers have complex fiber structures with spaces in between the fiber strands; synthetic fibers do not. Nylon does not have the pores needed for indigo to get stuck inside. This means that although nylon dyes quite well with many acid dyes, it should not be expected to take indigo dye.

Natural cellulose fibers such as linen, hemp, and cotten are best for dyeing with indigo, once you master the technique, which is more complex than that used for reactive dyes. Rayon is a reprocessed cellulose, so it generally dyes just like cotton (though it tends to produce brighter colors than cotton with reactive dyes). Bamboo fiber is a type of rayon. I'm not sure how well it dyes with indigo, but I think it will probably work. (In my experience, bamboo fiber dyes beautifully with reactive dyes, but I've heard that fabric made from it does not wear well, which is typical for other types of rayon.) Wool can be dyed with indigo if you use a recipe with a low enough pH not to damage the protein fiber.